The present invention relates to electronic devices, such as sensors and transistors, configured to be sealed in an atmosphere of reduced pressure or an atmosphere of inert gas.
Conventionally, electronic devices exhibiting high performance in an atmosphere of vacuum (or atmosphere of reduced pressure), such as infrared sensors and vacuum transistors, are typically sealed in a case of hermetic seal and ceramic or the like for use thereof. Such vacuum-packaged electronic devices include a so-called discrete type of device having a single sensor disposed therein and an integration type of device having a large number of sensors and transistors arranged in an array-like configuration.
On the other hand, it has been suggested to provide a smaller highly-integrated semiconductor device by sealing sensors, emitting elements or the like arranged in an array-like configuration into an atmosphere of vacuum, but not into a special case of ceramic or the like, by a packaging method employing a process for manufacturing semiconductor devices. For example, the official gazette of international publication No. WO 95/17014 describes a method for sealing a cell array-arranged area into a vacuum atmosphere. Therein, after a cell array of detectors for infrared radiation or the like or a cell array of emitting elements is formed on a first wafer, a second wafer is then placed on the first wafer with a predetermined spacing left between them, and the wafers are joined to each other by solder at the periphery of the cell array while the space between both the wafers is maintained under atmosphere of vacuum.
However, the technique described in the above official gazette has following problems.
First, when a large number of elements such as infrared detectors are arranged in an array-like configuration, it is difficult to make the whole joining portion at the periphery of the cell array completely flat, and thus an excessively high push pressure is inevitably required for thermal pressure bonding, thereby causing the possibility of broken wafers during joining, deteriorated vacuum caused by residual stress and device malfunctions.
Second, if a joining failure is caused at part of the joining portion for maintaining elements such as a large number of infrared detectors under vacuum, the vacuum is broken over the whole cell array, and thus the whole device becomes bad, thus resulting in a high percent of defective devices.
Third, when solder is used for the joining, the degree of vacuum in the inside space having the cell array placed therein can not be higher than a certain level because of outgassing of organic materials included in the solder paste. Therefore, there has been the possibility that an increase in the sensitivity of infrared sensors can not be expected, for example.